The invention relates to a method for producing an image from a video data stream and a system for projecting images from a video data stream. Specifically, the invention relates to the use of a spatial light modulator that uses a pulse-width-modulation scheme to modulate the light beam from one or more pulsed laser sources.
Projection display systems for the display of motion images from a video data stream have been known in the prior art. Typically, these systems have comprised a white light source, most notably a xenon arc lamp, illuminating one or more spatial light modulators to form the desired image. Lasers have been known to be an attractive alternative to arc lamps for projection displays. Potential advantages are wide color gamut featuring very saturated colors, simple, low-cost, efficient optical systems, and higher contrast with some spatial light modulators. Potential disadvantages of laser-based projectors are speckle and the lack of economical high-power lasers currently available at the appropriate visible wavelengths.
There is a limited selection of lasers that can be considered for use in cinema projection display systems. Gas lasers (argon-ion for blue and green emission and krypton for red emission) have been considered, but are undesirable due to the significant cooling and power required, and the limited selection of wavelengths for the color primaries. Diode lasers are another potential technology, but current materials cannot directly emit blue or green light at an appropriate wavelength or power. Frequency-doubled diode lasers are an alternative, but are not sufficiently advanced to consider for a cinema application.
Diode-pumped solid state lasers are another class of lasers appropriate for consideration. The laser transitions actually produce light in the infrared portion of the spectrum. However, it is well-known in the prior art that nonlinear optics can be used to upconvert the infrared light into visible light. Furthermore, a laser with an optical parametric oscillator can be used to generate one or more intermediate wavelengths from which red, green, and blue light can be produced through upconversion. This type of system has been disclosed by Wallenstein in U.S. Pat. Nos. 5,828,424 issued Oct. 27, 1998 and 6,233,025 issued May 15, 2001; by Nebel in U.S. Pat. No. 6,233,089 issued May 15, 2001; and by Moulton in U.S. Pat. No. 5,740,190 issued Apr. 14, 1998. The conversion efficiency in the nonlinear optical system required for the generation of red, green, and blue light can be made very high if a pulsed solid-state laser source is used. For example, Wallenstein discloses the use of a mode-locked laser oscillator while Moulton teaches the use of a Q-switched laser oscillator.
Turning to the spatial light modulator, a large variety of modulators have been used. A continuous illumination source, such as an arc lamp or a continuous-wave laser, allows significant flexibility in the modulation scheme used with the modulator. Some spatial light modulators utilize a pulse-width modulation scheme in order to modulate a light beam with image data. Each individual pixel in the array can be switched into an xe2x80x9conxe2x80x9d state that directs incident light to the screen for a time given by a multiple of a least-significant-bit (LSB) time. Tone scale is achieved in an individual pixel by controlling the ratio of the on times to the total time available for display of the pixel. Examples of spatial light modulators that can be operated in a pulse-width-modulation scheme are electromechanical grating devices, such as the conformal Grating ElectroMechanical System (GEMS) from Eastman Kodak Company and the Grating Light Valve (GLV) from Silicon Light Machines, micromirror array devices such as the Digital Light Processing (DLP) chip from Texas Instruments, and liquid crystal light valves.
Pulse-width-modulation is a desirable method of encoding digital image data in a display system. An advantage of pulse-width-modulation is that the application of the signal does not require a well-controlled voltage level from an analog voltage source. Hence, the modulation scheme is closer to being truly digital in that noise in the modulation process is determined by more easily controlled factors such as timing jitter. Additional advantages are reduced susceptibility to electrical crosstalk from adjacent pixels, to noise from electromagnetic interference, and to drifts from temperature variations.
A disadvantage of pulse-width-modulation is the difficulty with which gamma correction can be performed. Poynton, in Chapter 5 of A Technical Introduction to Digital Video, explains that due to the nonlinearity in the human visual response to luminance, the bit spacing between adjacent dark bit levels is required to be smaller than that between adjacent bright bit levels. Thus, when using a pulse-width modulation scheme, approximately 3-4 linear bits are lost in the gamma correction process. The result is that for a high-quality image,  greater than 12 linear bits/color are required, with 13-14 being optimum.
A second disadvantage arises when a periodically-pulsed laser beam illuminates an ideal modulator. In this scenario, the application of a varying pulse width to the modulator acts to vary the number of laser pulses that are passed through to the screen. This is known as pulse number modulation. A problem arises if the time between laser pulses is longer than the LSB time of the spatial light modulator. In this event, the bit depth of the image is limited by the laser pulse repetition rate rather than the LSB time, and therefore the image is not faithfully reproduced. With a scanned linear light-valve display, this phenomenon could result in banding artifacts to which human observers are very sensitive.
The problem of the laser pulse period being longer than the LSB time of the modulator can easily occur. For example, stability requirements in mode-locked lasers can limit many desirable laser systems to repetition rates of  less than 200 MHz, or 5 ns laser pulse period. However, a linear light valve system with 2000 scanned lines of resolution at 48 frames per second and 13 linear bits per color per frame would produce an LSB time of approximately 1 ns, so the achieved tone scale would be laser limited. As another example, a Q-switched laser is practically limited to repetition rates of  less than 100 kHz (for a minimum of 10 xcexcs between laser pulses), whereas an area spatial light modulator system with 13 linear bits per color per frame would produce an LSB time of approximately 2 xcexcs.
Takeuchi et al in xe2x80x9cLaser Digital Cinema(trademark),xe2x80x9d (Projection Displays VII, SPIE v. 4294, pp. 28-35 (2001)) discuss the use of a Q-switched RGB laser with a pulse-width-modulated micromirror array for a laser projection display system. However, the tone scale in their investigation was limited to 10 linear bits/color or less because of the switching speed of the modulator. Further, the laser pulses were required to be carefully synchronized with the modulation signal.
The object of the invention is to provide a display system that contains a pulsed laser source and a pulse-width-modulated light valve wherein the laser pulse repetition rate does not significantly limit the bit depth. The system does not produce a loss of tone scale or increase in noise in the image when the time between emitted laser pulses is longer than the least-significant-bit (LSB) time.
There is a need, therefore, for an improved method and system for image display using a pulsed laser source that can be used with a pulse-width-modulated light valve in a display system.
The need is met according to the present invention by providing a light-modulating system that includes: a light source for emitting a sequence of light pulses at a period Tp; a light modulator having a variable-width switching profile with rise and fall times Tr and Tf respectively, wherein the greater of Tr and Tf is greater than Tp, the light modulator modulating the sequence of light pulses; and a controller for controlling the width of the variable-width switching profile.
The illumination system of the present invention has the advantages that the use of a pulsed laser source does not significantly limit the bit depth and does not result in a loss of tone scale or increase in noise in the image when the time between emitted laser pulses is longer than the least-significant-bit (LSB) time.